Right now, when someone receives post-doctoral fellowship income, it's included in their income for purposes of calculating their taxes, so it's effectively taxed like employment income. However, there's a historical anomaly where it was not included in earned income for the purposes of calculating RRSP room, so the measure is really intended to provide fairness in giving someone RRSP room in a situation where they're paying tax on that income.
On May 3rd, 2022. See this statement in context.